1. Field of the Invention
The present invention relates to balanced positioning systems, such as may be used to position a moveable object in at least three degrees of freedom. More particularly, the invention relates to the use of such a balanced positioning system in lithographic projection apparatus comprising:
an illumination system for supplying a projection beam of radiation;
a first object table for holding patterning means capable of patterning the projection beam according to a desired pattern;
a second object table for holding a substrate; and
a projection system for imaging the patterned beam onto a target portion of the substrate.
2. Description of the Related Art
The term xe2x80x9cpatterning meansxe2x80x9d should be broadly interpreted as referring to means that can be used to endow an incoming radiation beam with a patterned cross-section, corresponding to a pattern that is to be created in a target portion of the substrate; the term xe2x80x9clight valvexe2x80x9d has also been used in this context. Generally, the said pattern will correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit or other device (see below). Examples of such patterning means include:
A mask held by said first object table. The concept of a mask is well known in lithography, and its includes mask types such as binary, alternating phase-shift, and attenuated phase-shift, as well as various hybrid mask types. Placement of such a mask in the projection beam causes selective transmission (in the case of a transmissive mask) or reflection (in the case of a reflective mask) of the radiation impinging on the mask, according to the pattern on the mask. The first object table ensures that the mask can be held at a desired position in the incoming projection beam, and that it can be moved relative to the beam if so desired.
A programmable mirror array held by a structure, which is referred to as first object table. An example of such a device is a matrix-addressable surface having a viscoelastic control layer and a reflective surface. The basic principle behind such an apparatus is that (for example) addressed areas of the reflective surface reflect incident light as diffracted light, whereas unaddressed areas reflect incident light as undiffracted light. Using an appropriate filter, the said undiffracted light can be filtered out of the reflected beam, leaving only the diffracted light behind; in this manner, the beam becomes patterned according to the addressing pattern of the matrix-addressable surface. The required matrix addressing can be performed using suitable electronic means. More information on such mirror arrays can be gleaned, for example, from U.S. Pat. Nos. 5,296,891 and 5,523,193, which are incorporated herein by reference.
A programmable LCD array, held by a structure which is referred to as first object table. An example of such a construction is given in U.S. Pat. No. 5,229,872, which is incorporated herein by reference.
For purposes of simplicity, the rest of this text may, at certain locations, specifically direct itself to examples involving a mask; however, the general principles discussed in such instances should be seen in the broader context of the patterning means as hereabove set forth.
For the sake of simplicity, the projection system may hereinafter be referred to as the xe2x80x9clensxe2x80x9d; however, this term should be broadly interpreted as encompassing various types of projection system, including refractive optics, reflective optics, and catadioptric systems, for example. The illumination system may also include components operating according to any of these design types for directing, shaping or controlling the projection beam of radiation, and such components may also be referred to below, collectively or singularly, as a xe2x80x9clensxe2x80x9d. In addition, the first and second object table may be referred to as the xe2x80x9cmask tablexe2x80x9d and the xe2x80x9csubstrate tablexe2x80x9d, respectively.
Lithographic projection apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, the patterning means may generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (comprising one or more dies) on a substrate (silicon wafer) that has been coated with a layer of radiationsensitive material (resist). In general, a single wafer will contain a whole network of adjacent target portions that are successively irradiated via the projection system, one at a time. In current apparatus, employing patterning by a mask on a mask table, a distinction can be made between two different types of machine. In one type of lithographic projection apparatus, each target portion is irradiated by exposing the entire mask pattern onto the target portion at once such an apparatus is commonly referred to as a wafer stepper. In an alternative apparatusxe2x80x94commonly referred to as a step-and-scan apparatusxe2x80x94each target portion is irradiated by progressively scanning the mask pattern under the projection beam in a given reference direction (the xe2x80x9cscanningxe2x80x9d direction) while synchronously scanning the substrate table parallel or anti-parallel to this direction; since, in general, the projection system will have a magnification factor M (generally  less than 1), the speed V at which the substrate table is scanned will be a factor M times that at which the mask table is scanned. More information with regard to lithographic devices as here described can be gleaned, for example, from U.S. Pat. No. 6,046,792, incorporated herein by reference.
In general, apparatus of this type contained a single first object (mask) table and a single second object (substrate) table. However, machines are becoming available in which there are at least two independently movable substrate tables; see, for example, the multi-stage apparatus described in U.S. Pat. No. 5,969,441 and U.S. Ser. No. 09/180,011, filed Feb. 27, 1998 (WO 98/40791), incorporated herein by reference. The basic operating principle behind such a multi-stage apparatus is that, while a first substrate table is underneath the projection system so as to allow exposure of a first substrate located on that table, a second substrate table can run to a loading position, discharge an exposed substrate, pick up a new substrate, perform some initial metrology steps on the new substrate, and then stand by to transfer this new substrate to the exposure position underneath the projection system as soon as exposure of the first substrate is completed, whence the cycle repeats itself; in this manner, it is possible to achieve a substantially increased machine throughout, which in turn improves the cost of ownership of the machine.
In a known lithographic apparatus, the drive unit of the positioning device for the substrate table comprises two linear Y-motors each of which comprises a stator extending parallel to the Y-direction and secured to a base of the positioning device, and a translator (Y-slider) movable along the stator. The base is secured to the frame of the lithographic device. The drive unit further comprises a linear X-motor that comprises a stator extending parallel to the X-direction and a translator (X-slider) which can be moved along the stator. The stator of the X-motor is mounted on an X-beam that is secured, near its respective ends, to the translators (Y-sliders) of the linear Y-motors. The arrangement is therefore H-shaped, with the two Y-motors forming the uprights and the X-motor forming the cross-piece, and this arrangement is often referred to as an H-drive.
The driven object, in this case the substrate table, can be provided with a so-called air foot. The air foot comprises a gas bearing by means of which the substrate table is guided so as to be movable over a guide surface of the base extending at right angles to the Z-direction.
In a lithographic apparatus, reactions on the machine frame to acceleration forces used to position the mask (reticle) and substrate (wafer) to nanometer accuracies are a major cause of vibration, impairing the accuracy of the apparatus. To minimize the effects of vibrations it is possible to provide an isolated metrology frame, on which all position sensing devices are mounted, and to channel all reaction forces to a so-called force or reaction frame that is separated from the remainder of the apparatus.
In an alternative arrangement, the reaction to the driving force is channeled to a balance mass, which is normally heavier than the driven mass which is free to move relative to the remainder of the apparatus. The reaction force is spent in accelerating the balance mass and does not significantly affect the remainder of the apparatus. Balance masses moveable in three degrees of freedom in a plane are described in WO 98/40791 and WO 98/28665 (mentioned above), as well as U.S. Pat. No. 5,815,246.
EP-A-0,557,100 describes a system which relies on actively driving two masses in opposite directions so that the reaction forces are equal and opposite and so cancel out. The system described operates in two dimensions but the active positioning of the balance mass necessitates a second positioning system of equal quality and capability to that driving the primary object.
An object of the present invention is to provide a balancing system that is readily extendable to multiple degrees of freedom and is usable with various different drive mechanisms.
According to the present invention there is provided a lithographic projection apparatus comprising:
an illumination system for supplying a projection beam of radiation;
a first object table for holding patterning means capable of patterning the projection beam according to a desired pattern;
a second object table for holding a substrate; and
a projection system for imaging the patterned beam onto a target portion of the substrate; and
a balanced positioning system capable of positioning at least one of said object tables in more than three degrees of freedom, the positioning system comprising:
at least one balance mass;
bearing means for movably supporting said balance mass;
coarse positioning means for positioning said object table in first to third degrees of freedom, said three degrees of freedom being translation in first and second directions and rotation about a third direction, said first, second and third directions being substantially mutually orthogonal; and
fine positioning means for positioning said object table in at least a fourth degree of freedom substantially orthogonal to said first, second and third degrees of freedom, said coarse and fine positioning means being arranged so that reaction forces from said coarse and fine positioning means are channeled to said balance mass; characterized in that:
said balance mass is supported by said bearing means so as to be substantially free to move in at least said fourth degree of freedom.
The long stroke (coarse) positioning system of a lithography apparatus is normally arranged to position the apparatus in X, Y and Rz degrees of freedom whilst a short stroke (fine) positioning system provides higher-precision positioning over all 6 degrees of freedom (i.e. X, Y, Z, Rz, Ry, and Rx). The positioning movements of the short stroke positioning system can be a source of undesirable vibrations in the apparatus. These movements are often of much higher frequency than movements of the long stroke positioning system and can involve high accelerations so that the reaction forces are large, even though the moving mass is smaller. By arranging for the reaction forces of the fine positioning means to be channeled to the balance mass, which is free to move in at least one additional degree of freedom, directly or via the coarse positioning means, the present invention ensures that all reaction forces are confined to the balanced positioning system and vibrations in the remainder of the apparatus are minimized.
The balance mass may be a single body moveable in at least four degrees of freedom or may be made up of several parts separately moveable in one or more degrees of freedom. For example, in an embodiment of the invention a first part of the balance mass is a frame moveable in the first to third degrees of freedom (e.g. X, Y and Rz) and surrounding the object table whilst a second part of the balance mass is disposed underneath the object table and is moveable in at least the fourth degree of freedom (e.g. Z).
According to a further aspect of the present invention there is provided a lithographic projection apparatus comprising:
an illumination system for supplying a projection beam of radiation;
a first object table for holding patterning means capable of patterning the projection beam according to a desired pattern;
a second object table for holding a substrate; and
a projection system for imaging the patterned beam onto a target portion of the substrate; and
a balanced positioning system capable of positioning at least on of said object tables in at least three degrees of freedom, the positioning system comprising:
at least one balance mass;
bearing means for supporting said balance mass so as to be substantially free to move in said three degrees of freedom; and
driving means for acting directly between said object table and said balance mass to position said object table in said three degrees of freedom; characterized in that:
said balance mass comprises a generally rectangular frame having its sides generally parallel to said first and second directions, and a central opening in which said object table is at least partly disposed.
With the balance mass in the form of a rectangular frame, the drives forming the uprights of a so-called H-drive arrangement can easily be integrated into the sides of the frame ensuring that the reaction forces all act directly between balance mass and driven object table. Also, because the driven object table sits within the central opening of the balance frame the distance in the Z-direction between the centers of gravity of the balance frame and the driven mass is reduced.
To reduce the excursions of the balance mass, and hence the overall footprint of the apparatus, it is preferred that the balance mass is considerably more massive, preferably at least five times, than the positioned object. In this regard, all masses that move with the balance mass are considered part of it and all masses that move with the positioned object are considered part of that.
It should be noted that in embodiments of the invention according to either of the aspects described above, multiple object (mask or substrate) tables may be provided and the reaction forces to the drive forces of two or more tables may be directed to a common balance mass or masses.
According to yet a further aspect of the present invention there is provided a lithographic projection apparatus comprising:
an illumination system for supplying a projection beam of radiation;
a first object table for holding patterning means capable of patterning the projection beam according to a desired pattern;
a second object table for holding a substrate; and
a projection system for imaging the patterned beam onto a target portion of the substrate; and
a balanced positioning system capable of positioning at least on of said object tables in at least two degrees of freedom, the positioning system comprising:
at least one balance mass;
bearing means for movably supporting said balance mass;
positioning means for positioning said object table in at least first and second degrees of freedom, said first to second degrees of freedom being translations in first and second directions that are substantially orthogonal, said positioning means comprising coarse and fine positioning means and being arranged so that reaction forces from said positioning means are channeled to said balance mass; characterized in that:
said coarse positioning means comprises a planar electric motor having a translator mounted to said object table and a stator extending parallel to said first and second directions and mounted to said balance mass.
The forces exerted by the planar motor will be channeled directly to the balance mass in the first and the second direction as opposed to an H-drive arrangement. In an H-drive arrangement forces may be channeled indirectly to the balance mass, since the object table is driven by an X-slider over an X-beam in the X-direction and the X-beam and object table are driven in the Y-direction by two Y-direction linear motors with corresponding sliders mounted to both ends of the X-beam. Only the beams of the Y-linear motors are mounted to the balance mass. Forces exerted in the X-direction by the X-motor will be channeled indirectly via the X-beam and the Y-direction linear motors to the balance mass. When a planar motor is used reaction forces in both the X-direction and the Y-direction are directly channeled to the balance mass. Further, with the stator (e.g. a magnet array) mounted to the balance mass, the mass of the balance mass is desirably increased to reduce its movement range.
In a vacuum environment it may be advantageous to use the planar motor also to levitate the object table because it will be difficult to use a gas bearing to levitate the object table in a vacuum environment. The planar motor may also be used to rotate the object table around a third direction being mutually orthogonal to said first and second direction.
The magnate levitation of the planar motor provides for a frictionless bearing allowing the balance mass to, freely move in first and second directions and rotate around the third direction. The balance mass may also be movable in the third direction and/or rotatable around one or both of the first and second directions such that it provides balancing in more than three degrees of freedom. For this purpose the balance mass may be supported by supports having a low stiffness in the third direction. The balance mass may be provided with upstanding walls to raise the center of gravity of the balance mass to the same level in the third direction as the center of gravity of the object table.
According to a further aspect of the invention there is provided a method of manufacturing a device using a lithographic projection apparatus comprising:
an illumination system for supplying a projection beam of radiation;
a first object table for holding patterning means capable of patterning the projection beam according to a desired pattern;
a second object table for holding a substrate; and
a projection system for imaging the patterned beam onto a target portion of the substrate; the method comprising the steps of:
providing a substrate provided with a radiation-sensitive layer to said second object table;
providing a projection beam of radiation using an illumination system;
using patterning means to endow the projection beam with a pattern in its cross-section;
projecting the patterned beam of radiation onto target portions of said substrate;
wherein during or prior to said projecting step at least one of said object tables is moved in first to third degrees of freedom by coarse positioning means and in at least a fourth degree of freedom by fine positioning means and, during such movement, reaction forces in said first to third degrees of freedom are exerted on a balance mass;
characterized by the further step of:
channeling reaction forces in said fourth degree of freedom to said balance mass.
In a manufacturing process using a lithographic projection apparatus according to the invention a pattern (e.g. in a mask) is imaged onto a substrate which is at least partially covered by a layer of radiation-sensitive material (resist). Prior to this imaging step, the substrate may undergo various procedures, such as priming, resist coating and a soft bake. After exposure, the substrate may be subjected to other procedures, such as a post-exposure bake (PEB), development, a hard bake and measurement/inspection of the imaged features. This array of procedures is used as a basis to pattern an individual layer of a device, e.g. an IC. Such a patterned layer may then undergo various processes such as etching, ion-implantation (doping), metallization, oxidation, chemo-mechanical polishing, etc., all intended to finish off an individual layer. If several layers are required, then the whole procedure or a variant thereof, will have to be repeated for each new layer. Eventually, an array of devices will be present on the substrate (wafer). These devices are then separated from one another by a technique such as dicing or sawing, whence the individual devices can be mounted on a carrier, connected to pins, etc. further information regarding such processes can be obtained, for example, from the book xe2x80x9cMicrochip Fabrication: A Practical Guide to Semiconductor Processingxe2x80x9d, Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN 0-07-067250-4.
Although specific reference may be made in this text to the use of the apparatus according to the invention in the manufacture of ICs, it should be explicitly understood that such an apparatus has many other possible applications. For example, it may be employed in the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, liquid-crystal display panels, thin-film magnetic heads, etc. The skilled artisan will appreciate that, in the context of such alternative applications, any use of the terms xe2x80x9creticlexe2x80x9d, xe2x80x9cwaferxe2x80x9d or xe2x80x9cdiexe2x80x9d in this text should be considered as being replaced by the more general terms xe2x80x9cmaskxe2x80x9d, xe2x80x9csubstratexe2x80x9d and xe2x80x9ctarget areaxe2x80x9d or xe2x80x9ctarget portionxe2x80x9d, respectively.
In the present document, the terms illumination radiation and illumination beam are used to encompass all types of electromagnetic radiation or particle flux, including, but not limited to, ultraviolet radiation (e.g. at a wavelength of 365 nm, 248 nm, 193 nm, 157 nm or 126 nm), EUV, X-rays, electrons and ions.
The invention is described below with reference to an orthogonal reference system based on X, Y and Z-axes. The Z direction may be referred to as vertical but this should not, unless the context demands, be taken as implying any necessary orientation of the device.